Aerospace applications that require precise surface shapes for pointing or focusing signals are widespread. In satellite communications, microwave antennas require smooth surfaces of certain geometric shapes for long distance information transmissions. Although these surfaces can be manufactured on earth with precise tolerances, in space, such environmental hazards as large thermal gradients and space debris cause unacceptable surface distortions. From a mechanical viewpoint, today's space-based antennas are fixed-shape structures that deploy. Among the antenna's performance objectives, the most significant are that small thermally induced deformations be maintained, that the system mass be minimal and that the antenna be deployed reliably. Toward that end, low density, high strength, low coefficient of expansion materials are selected, shrouds reduce thermal gradients, and configurations are optimized to reduce deformations. Still, the inherent trade-offs between low mass and small deformations in a thermally varying environment limit the performances achieved. Also, mechanical deployment mechanisms continue to suffer in their performance. This is due to the trade off between the desire to minimize clearances to reduce slop, and the desire to maximize clearances to reduce the chance of contact surface sticktion. (See "1.4 Billion Dollar Galileo Mission Appears Crippled", Washington Post, Dec. 18, 1991).
Previous attempts to develop antenna reflectors using electrostatics involved suspending an electrically conductive material in a support structure. See for example U.S. Pat. No. 4,093,351 and U.S. Pat. No. 4,571,594. The support structure of these antennas defined the shape of the periphery of the antenna and the material was then electrostatically tensioned against this structure. These efforts exhibited the same limitations as mechanically shaped antennas in the thermally varying space environment in that the relative size of the tension forces quickly overcame the electrostatic forces to limit the ability to shape the conductive material. Furthermore, these efforts were limited in that the support structure must be moved to retarget the antenna and any changes in the swath of the antenna are limited.
Indeed, the varying shape capability of electrostatically shaped membranes is greatly expanded when the limitations of an external supporting structure are removed and when the characteristics of an antenna employing the teachings of the present invention are employed. The expanded capability allows the antenna to transmit and to receive information from ground swaths that vary in size, to retarget, to refocus, to vary focal length and to rapidly scan.
It is, therefore, one objective of the present invention to provide an electrostatically shaped membrane suitable for use in space-based electrostatic antennas, capable of varying its shape and electrostatic deployment unlike present fixed shape mechanically deployed antennas.
It is a further objective of the present invention to provide an ultra low weight antenna by providing an electrostatically shaped membrane which may be retargeted, refocused, the ground swath varied or scanned rapidly without disturbing the satellite dynamics.
It is an additional objective of the present invention to provide an electrostatically shaped membrane suitable for electrostatic deployment which does not suffer from the inherent trade-offs that limit the performance of mechanical mechanisms. Increased reliability in deployment enables deployment and retractment to be carried out repeatedly and thereby allows for a stowed antenna which is protectable from such environmental hazards as high dose radiation exposure, space debris and destructive interception.
It is another objective of the present invention, and one of particular practical value, to provide an electrostatically shaped membrane for use in an antenna which may serve as a multifunction system replacing functions otherwise carried out separately by as many as five or more antennas.